We have learned an enormous amount in the past budget period about the mechanism and biology of[unreadable] one of the major components of the human genome, the LI (LINE) retrotransposon. This remarkable[unreadable] element is responsible, directly or indirectly, for about one third of our genome by weight; its reverse[unreadable] transcriptase ORF (ORF2) is the most abundant ORF in the human genome. Recent gains in[unreadable] understanding how this element works result from technical breakthroughs. We conceived of,[unreadable] developed and exploited a new synthetic retrotransposon that is approximately 200-fold more efficient than a native[unreadable] element. We will use human tissue culture cell, in vitro and in silico systems to analyze the molecular[unreadable] mechanisms by which the LI element replicates and inserts itself into new sites. We will also[unreadable] investigate the mechanism and consequences of a newly discovered expression regulatory phenomenon[unreadable] we discovered in LI, namely that its ORF sequences block transcriptional elongation. In the opposite[unreadable] orientation they lead to premature polyadenylation. These combined effects may affect the expression[unreadable] of many human genes and the responsible insertion polymorphisms may well underlie complex traits[unreadable] such as cancer susceptibility. These hypotheses will be tested. Finally, we are building a transgenic[unreadable] synthetic retrotransposon mouse model that should provide technology for a) making genome wide[unreadable] knockout mutation collections and b) discovery of tumor suppressor genes in leukemia and other[unreadable] cancers.